Exploding pen joke

Fountain pens were common in the mid 20th century. When you lend this pen to a friend, there is a loud explosion as they take the top off. To ‘load’ this pen you need to pull back the spring-loaded metal plate and put a percussion cap underneath. Then carefully put the top of the pen in position, so stopping the spring-loaded metal plate from impacting the cap. When the top of the pen is next removed, the metal plate is free to flip round and land on the cap, causing it to explode. The advertisement is from a Davenports catalogue.